Umesh Shrestha

Water dynamics in rigid ionomer networks

The dynamics of water within ionic polymer networks formed by sulfonated poly(phenylene) (SPP), as revealed by quasi-elastic neutron scattering (QENS), is presented. These polymers are distinguished from other ionic macromolecules by their rigidity and therefore in their network structure. QENS measurements as a function of temperature as the fraction of ionic groups and humidity were varied have shown that the polymer molecules are immobile while absorbed water molecules remain dynamic. The water molecules occupy multiple sites, either bound or loosely constrained, and bounce between the two. With increasing temperature and hydration levels, the system becomes more dynamic. Water molecules remain mobile even at subzero temperatures, illustrating the applicability of the SPP membrane for selective transport over a broad temperature range.

Self-assembly of a semi-fluorinated diblock copolymer in a selective solvent

The self-assembly of a highly incompatible siloxane containing semi-fluorinated diblock copolymer, polytrifluoro propyl methylsiloxane-b-polystyrene (SiF-PS), in toluene, a selective solvent for polystyrene, was studied using Small Angle Neutron Scattering. Incompatibility is often enhanced by inserting fluorinated segments into one of the blocks and as a result not only the interchain interactions are changed but also the rigidity of the blocks. Herein the incorporation of siloxane into the backbone of a semi-fluorinated block maintains its flexibility and allows separation of the effects of direct interactions due to fluorine atoms from those of rigidity. Measurements were carried out in dilute solutions below 1 wt%, at volume fractions ϕSiF ranging from 0.0 to 0.5. The high incompatibility of the SiF block drives aggregation at low volume fractions of the SiF block, where spherical core–Gaussian shell aggregates are detected at ϕSiF = 0.16. In the symmetric SiF-PS complex fluid, elongated micelles were observed. The micelles exhibited unique temperature stability in comparison with the aggregates formed by diblock-copolymers in the lower segregation regime. As the temperature increases the micelles dissociate into free chains to form unimolecular micelles.